Growth of microalgae phototrophically in outdoor open systems subjects the culture of microalgae to multiple challenges not faced by the higher cost closed fermentation systems, such as but not limited to larger loss of gases at the liquid/gas interface and contamination by a variety of microorganisms. Airtight fermentation systems offer better contamination control for heterotrophic systems, thus lending such systems to become the primary production method for many commercial microalgae products. However, fermentation systems are costly to install and operate, and light is limited supply to support a meaningful photosynthetic activity necessary for some algal products.
With the high cost of commercial fermentations systems, methods to achieve the higher growth rates of closed heterotrophic systems have been investigated using the lower cost open systems through mixotrophic cultivation. However, using organic carbon in an open culture has created challenges to the survival of microalgae due to the level of contamination that results.
While contamination exists in open phototrophic microalgae cultures, the introduction of organic carbon results in not only an increased amount of contamination, but also different contaminating organisms than those found in phototrophic cultures. The differences in the contaminating organisms include, but are not limited to, different sizes and rates of replication. For example, contamination in phototrophic cultures primarily comprise predatory organisms such as rotifers, while cultures with organic carbon experience more bacterial contamination. Therefore, the methods traditionally used for contamination control in open phototrophic cultures have not been found to be effective due to these differences, and more effective methods need to be developed to control contamination in open cultures containing organic carbon.